Optical biosensors are powerful tools used in the analysis of biomolecular interactions, a key part of the drug discovery process. They can not only provide detailed information on the binding affinity and kinetics of the biomolecular interaction, but can also be exploited for the detection of bacteria and viruses for medical diagnosis, environmental monitoring, and homeland security.
A biosensor generally comprises a surface coated with biorecognition elements selected for their specific binding affinity with a molecule of interest (i.e., the molecule to be detected/measured). To be detected, binding must result in a physically detectable change. Optical biosensors are used to sense the local increase of refractive index that occurs near the surface as a result of the accumulation of the molecules of interest.
Optical resonator biosensors use optical resonators that exhibit sharp, i.e., high Q, optical resonances from their whispering gallery modes (WGM). Those resonances can be used for highly sensitive biodetection as they shift when small amounts of biomolecules attach to the resonator surface. As compared to planar devices, microresonator-based detectors take advantage of the recirculation of light, effectively increasing the interaction length with the sensing surface, thus yielding higher sensitivity.
Microsphere resonators made of dielectric material are particularly attractive because they can sustain high Q resonances and therefore provide more accurate results. In some cases, microspheres are placed on stems and are handled with micro-positioners. Unfortunately, it can be difficult to handle and position a microsphere in a desired manner in such circumstances because of the small size of the microsphere and the precision needed for its placement. In other cases, microspheres are permanently bonded to an optical substrate or fiber. While this can make it easier to handle and position the microsphere, such an arrangement is irreversible.
In view of the above discussion it can be appreciated that it would be desirable to have a way to easily handle and position a microsphere for optical biosensing.